Hi,
afaik there is no build-in mechanism for this.
A proposed workaround for this is:
1. on all ingress mark all pakets with an appropiate user-packet-class (not using at least one possible user-packet-class value),
2. on the egress: if a packet leaves the router which doesn´t have a user-packet-class as assigned by 1. it must be packet generated by the router internally, so control-traffic (PPP, PPPoE, ICMP, IGMP etc.). So set 802.1p / user-priority to the desired values.
Please let me know if this works for you.
Best Regards
Ulf